A Very Large VU Meter Indeed

It used to be a must-have on any hi-fi, a pair of moving coil meters or LED bar graphs, the VU meter. Your 1980s boombox would have had them, for example. VU, for “Volume Units”, is a measure of audio level, and the fashion for its visual measure in consumer audio equipment seems now to have largely passed.

The LED bar graph VU meters were invariably driven by the LM3915, a chip that contains a resistor ladder and a stack of comparators which can drive LEDs directly. [Juvar] has taken an LM3915, and used it to drive a set of opto-isolated triacs which in turn drive a stack of appropriately coloured mains LED bulbs concealed within an Ikea Vidja lamp. The result is a huge and very bright VU meter that is as much a lighting effect as it is a measure of sound level.

He’s posted a video of the lights in action, and we’ve placed it below the break. There is a cameo appearance from his cat, and one can’t escape the feeling that it is wasted on a small room and would be at its best before a dance floor. Still, it’s a neat lighting effect and a new use for a classic integrated circuit.

I think it has more to do with recording to audio cassettes, what nearly nobody does any more these days. They had only around 70dB of dynamic range, so you wanted to be sure to use this as good as possible. Today you record digitally, mostly on a computer, where you have a screen to display they audio level very easily.

Those chips have ten segments, this looks like less. But the real letdown is the diffusion between segments, with those washed together colors. It makes the whole display act like that string of resistors and LED’s that takes the place of the display chip when encountered in cheap stuff. Disc dividers, use old CD’s they reflect.

Be careful with how diffused LED projects look on video! I’m not sure if it’s the case for the VU meter featured in the video, given that I’ve never been in Juvar’s living room, but I built a light up dance floor a few years back and I know that on the video the individual floor cells looks FAR less diffused than how it looks to human eyes.

Ultimately it’s what it looks like in person, which in my case looks great, so I’ve never bothered to change the illumination scheme just to make videos of it look better. I’m betting that something very similar is going on with this project as well.

The LM3914 is linear, as the steps are equal (the ladder has 1K per step).
The LM3915 is ANALOG, with 3dB per step on it’s ladder.
The LM3916 is the TRUE VU display, and if you look at the datasheet, you’ll see the ladder has R values all over the place, to match a true VU meter response.

So, can we all STOP calling any flashing lights to music a “VU Display”, when it may not actually be that? Please?

@Martin–
VU meters were used long before audio cassettes were a consideration by anyone, for anything, and still are used to set, and monitor approximate audio levels to avoid overloading a downstream audio device. Wouldn’t be a bit surprised to find that VU meters were used in 1930’s radio stations and recording studios.

1)
he is right to say why AVERAGE non-techie/light-set-lovers would have andor use them andor expect them cuz thats just whats included when youve paid extra for the expensive “high-fi” version. its what differenciates the tape-recorder from the tape-deck, it also fits in with tape dubbing andor mixing, you cant get a good sound from Auto-Level-Control and need the level to adjust manually. so dancing people (people listening to mixtapes) happen to already HAVE one.

2)
all radio stations always have, and always will, have VU meters of several types for several purposes, and always will, as long as there are license/power/modulation/deviation/ect requirements, there will still be meters for high-power transmission of analog signals, although they might be a graphic on a “screen” now-a-days.

PS: many units only lit-up the meters when recording, but there was two options, 1) get a dual-deck and record-on-pause in one deck while listening to the other deck, or 2) if you HAVE a single-deck with meter and wish to use it while playing tape or radio, you need to hack it, which means sorting out the pinout of the spring-loaded circuit-board-mounted record/play mode switch, theres a lot of connections there, its the brains of the older non-computerized units.

The real issue is probably that there is no diode or rectifier in the signal path so the output is the whole ac signal with all of it’s high frequency components that ate too fast to have any meaning to the human eye apart from blurring all the segments together somewhat.